One of the instruments used in laboratories for acquiring a predetermined volume of a fluid is a pipet. For automated procedures a fluid is aspirated by the movement of a piston and no visual or other acknowledgement of the aspirated volume is obtained.
Due to several causes, the actually aspirated and/or dispensed volume does not always match the intended aspirated volume. Possible causes are:                the aspiration of particulate objects like blood clots fully, or partly blocking the tip's orifice during the full aspiration period or a fraction thereof,        a too small a volume in the container where the fluid is to be aspirated from; or        the aspiration of foam that may be present on top of the fluid in the container where the fluid is aspirated from.        
Techniques have been developed to detect possible conditions that may indicate deviations of the aspirated volume. In one such technique, the pipet is provided with a pressure sensor in or close to the tip of the pipet. The fluid is aspirated a first time and the pressure transient p(t) in the pipet is measured during aspiration. If the aspirated volume is correct, a pressure profile has been obtained that reflects a correct aspiration. This profile is then stored for later use. During subsequent aspirations of other fluids and using other tips, the corresponding pressure transient in the, possibly other, tip is measured and—after some form of data processing—compared to the profile of the first run. As long as the pressure stays within a predetermined bandwidth of the profile of the first run, the aspirated volume is assumed to be within an acceptable tolerance of the intended volume.
Drawback of this technique is that due to differences in for example the diameter of the orifice of the tip, variations of the pressure profiles are possible, depending on the specific pipet tip used and on the varying fluid properties, causing the acceptance bandwidth to be broad and the method to be insensitive.